1. Field of the Invention
The present invention relates to an image forming apparatus having a belt unit.
2. Description of the Related Art
Conventionally, configurations of stretching the inner surface of an endless belt using a plurality of stretching members have been known. Japanese Patent Application Laid-Open No. 2007-057953 discusses an image forming apparatus having a mechanism for moving at least one of a plurality of stretching members to a position where tension of a belt goes slack, and using the mechanism to cause the tension of the belt to be released.
In some cases, the image forming apparatus may not be used by users for a long time after the production of the image forming apparatus. For the above reason, configurations in which the tension of the belt is loosened at the time of manufacture, and the tension is applied to the belt at initial installation by a user have been known. For example, Japanese Patent Application Laid-Open No. 2007-286606 discusses an image forming apparatus having a mechanism for applying tension to a belt using power of a user when the user pulls a paper feed cassette of the image forming apparatus out of the apparatus body at initial installation.
However, in the known configurations, problems may occur in making the stretching member come in contact with the belt to set the tension of the belt at high level. In the configuration in Japanese Patent Application Laid-Open No. 2007-057953, one of the plurality of stretching members is moved while the relation with the other stretching members is kept in parallel, and the stretching members come in contact with the belt at the same time at the entire area in the axis direction. Accordingly, the impact in stretching the belt is large. Further, in controlling the positions of the stretching members using the cam discussed in Japanese Patent Application Laid-Open No. 2007-057953, due to a large impact generated when the stretching members come in contact with the belt, the cam may move. Then, the positions of the stretching members may not be kept at the regular positions.
As discussed in Japanese Patent Application Laid-Open No. 2007-286606, in the configuration the tension is applied to the belt using the user's power, depending on the power of the user, the impact due to the contact of the stretching members with the belt increases.
As described above, the large impact in stretching the belt may not only cause damage to the belt, but also cause an impact sound unpleasant to the users.